As is well known, electric motors are apparatuses used to convert electric energy into mechanic energy using electro-mechanical means.
The operation of an electric motor can be achieved by two principles discovered by mid-nineteenth century. The first principle is the one about induction discovered by Faraday, which says that if a conductor is moved through a magnetic field an electric current is induced in such conductor. The second principle was observed by Ampere and it recites that if a current passes through a conductor localized in the interior of a magnetic field, the latter applies a mechanical force on the conductor.
For the motors to be operational they are provided with to basic units: the inductor which creates the magnetic field and the shell or armature which is the structure supporting the conductors that cut the magnetic field and carry the motor excitation current.
Depending on the type of electric energy used by the motors, these are classified into direct current motors and alternating current motors. Emphasizing alternating current motors, the existence of two types should be noted: synchronous motors and induction motors (asynchronous). In synchronous motors the magnetic field current is supplied by an alternating current power source, while in induction motors the magnetic field current is supplied to its windings by magnetic induction.
It is exactly from this feature that the induction motor gets its name, since the currents flowing in the rotor are induced by the currents flowing in the stator. The rotor currents are induced by the action of magnetic fields generated by the stator winding without the existence of an electric connection between the stator circuit and the rotor.
The induction motor stator comprises a frame housing a magnetically active cylindrical annular structure, a stack of punched laminations of electrical steel with a winding set placed in internal equidistantly spaced slots.
The induction motor rotor is a magnetically active cylindrical structure (laminations stacking), mounted on a shaft. The rotor winding may be of two types: squirrel-cage rotor or winded rotor. Squirrel-cage type rotors comprise a series of conductive bars (aluminum or copper) disposed between carved slots on the rotor periphery and short-circuited on each end by short-circuit rings (aluminum or copper). The squirrel-cage winding may be formed either by melting, pressure injection or by a manufacturing process. The design variation of the rotor bars is a primary method for altering the motor torque-speed features.
It has to be noted that single-phase induction motors have ignition difficulties. To overcome such inconvenience, an igniting capacitor responsible for offsetting the current to achieve the torque required for the rotor to start rotating may be used. Likewise, a shadow winding (also known as auxiliary or igniting winding) can be used, which differs physically and electrically from the main or operation winding since the latter is formed by a thinner conductor and has more loops than the igniting winding.
Induction motors with auxiliary windings (also known as shaded-pole) have the advantage of having a simple construction, high reliability and strength, apart from having a low manufacturing cost. In contrast to other single-phase induction motor types, such motors do not require auxiliary parts such as centrifugal capacitors or switches, which can be interpreted as minimal maintenance.
Despite the squirrel-cage type single-phase induction motors not having a large igniting torque, they are used in various applications, such as: ventilators, centrifugal pumps, fuel pumps, dusty environments applications, compressors, washing machines and home appliances in general, etc.
In relation to the aforementioned, improvements in electric motors construction and configuration have been implemented through the years in order to achieve better results regarding their energy consumption, especially in those single-phase induction motors.
An example of the aforementioned is the shaded-pole single-phase induction motor disclosed in the U.S. Pat. No. 2,773,999 characterized for having a rotor comprised by a plurality of small ring-short-circuited bars; a stator with four protruding poles, each of them having an identical shape; said shape formed by two portions: a neck portion and a face portion; said face portion comprises a first and a second part. The first part has a slot where a shadow winding is placed. Each of the second parts has a semi-flat or divergent surface that attaches to the cylindrical partial portion of the face portion in a given point. The semi-flat surfaces are preferably tangential to the cylindrical surfaces and are extended to the end of the faces sides.
The rotor has preferably a very small air gap compared to the face portions. However, since the rotor is cylindrical, the air gap is uniform in the adjacent zone to the first parts and adjacent to the main central portion of the face portions. However, a different air gap is formed adjacent to the semi-flat surfaces, which continuously increases toward the faces ends. Also, such semi-flat surfaces are preferably at 50 electrical degrees.
The afore mentioned motor presents the inconvenience of a circular geometric shape of the stator, whereby a great amount of material is wasted when performing the punching to obtain the final form, as it is part of a square-shape lamination. Another inconvenience is that the pockets formed between one pole and the other are ovoid-shaped, which limits the useful space to place the main winding of each pole.
Another example of the previously mentioned is the motor disclosed in U.S. Pat. No. 2,149,569 that is of the shaded-pole reversible type, which in combination comprises a stator comprising a plurality of magnetic material laminations; such laminations being open inside their marginal edges in order to define the rotor pocket surrounded by a plurality of symmetrically disposed rectangular-shaped poles; a rotor mounted on such pocket for inducing the rotation, which includes a magnetic cylindrical core that has a winding and a squirrel-cage configuration; the faces of such poles are integrally formed by such laminations in order to have a coherent shape with said rotor contour; U-shaped linking members of portions of highly permeable magnetic metal sheets alternately brazing and firmly fastening each pole with the legs of such linking members; the leg faces of such linking members coincide in shape and contour with such rotor; means for permanently shading some of the poles and means for energizing the main windings placed in each of the poles.
However, although the stator of the aforementioned motor has a square shape, there is not an optimization of the space pretended for the main windings, thus to achieve the necessary magnetic field a wire with a smaller size is used. Whereby such motor does not have efficient energy consumption, because the smaller a conductor area is, the bigger its resistance to the electric current flow will be.
Another shaded-pole induction motor of the state of the art is described in U.S. Pat. No. 2,591,117 which consists of a squirrel-cage rotor, a concentrically located stator about said rotor; said stator being symmetric and being divided into at least four protruding opposing poles which form radial air gaps with said rotor, each pole being separated from its adjacent pole by circumferential air gaps circumferential of at least 5% and no more than 15% of the length of the arched face of one of said poles, each of said poles having a portion of the stepped face to provide a further radial gap to gap; the cumulative curvature of said stepped portions of the pole faces being at least 20% of a full circle with which said pole faces coincide; the reluctance of said circumferential air gaps being substantially greater than the reluctance of the additional air gaps.
However, it presents the same inconveniences of the aforementioned motors; the stator configuration is not optimal because there is much punching material waste in addition to lacking a suitable surface for thermal energy transfer between the stator and the environment.
The motor pertaining to U.S. Pat. No. 3,697,842 is a single-phase induction motor which includes a stator core that has at least one pair of poles. Said poles comprise a first and a second portion, the first portion having a shadow coil on the remote side of the second portion with which sections with and without shadow are formed from the first portion of the pole. A first coil braces each of the poles and a second coil only braces the first portion of each pole. The first coils are connected in series to form a first winding and the second coils are also connected in series to form a second winding; both the first and second winding are connected in series.
A first speed responding switch is provided, which shows a first and a second position. When being in its first position, the switch is coupled to the first winding in order to energize by a source of alternating current to the motor and begin operating. The switch also operates while being on its second position when the motor reaches certain speed wherein the first winding and the second winding are coupled in series to energize by the source.
The aforementioned induction motor has the disadvantage of using a centrifugal switch to enable the first winding and the second winding once that such motor is operating. The use of such additional components such as the switch increases the final cost apart from requiring periodic maintenance in order to guarantee its correct operation.
In U.S. Pat. No. 5,036,237 another example of electric motor that comprises a shaded-pole electric motor that has a stator and a rotor which interact by an air gap can be found. Said stator has a protruding pole with an arrow neck and a wide pole base. The slot of the shadow coil extends into the protruding pole from the air gap and defines a portion of the pole shadow. The exposed side of the shadow portion is relatively continuous, as it comprises a shadow coil that has a side within the slot of the shadow coil and other side close to the pole opening but away from the exposed side. This enables isolation of the surface of the protruding pole in order to receive the stator winding.
It has the disadvantage that when the protruding poles are formed by two sections separate from one another by a certain distance, a greater area is obtained for the windings but the pole amount that can be radially disposed in the stator is greatly limited. When the stator is punched there is a waste of punched material that is not being used on the following steps of motor construction.
In the aforementioned patents, reference is made to a stator and a rotor wherein the stator is a module formed by a set o steel sheets stack, which have been punched or cut with a specific design in order to house, inside the pockets that are being punched, the magnet wire loops (winding), said pockets are characterized for being provided in a diagonal position relative to a square horizontal and vertical symmetrical axes. The stator lamination in this induction motors can be squared or round; however, the design of this type of motors presents the following inconveniences:                When the lamination is square type, the excess material of the corners does not help to improve the magnetic flow of the motor, and when the lamination is of circular type, it presents a sheet waste on the contour of its corners, considering that the punching is performed on a square sheet.        When the punched pockets are arranged in an aligned position with the corners of the square electric sheet, they don't allow increasing the free space area among the protruding poles in which the coil is inserted.        The useful area in the square lamination of a standard motor of the previous art (3.3 frame) and which forms the punched pockets for housing the “coil” presents an area of about 406.62 mm2 which limits the area capacity for the winding.        Similarly, the useful area of the round lamination has a further reduced area, relative to the square, of about 318.18 mm2.        
Regarding the rotor, it consists of a set o electric grade steel sheets, with a smaller diameter than the internal diameter of the stator and with multiple punched slots, where the conductive bars are housed, which form the squirrel-cage of the previous art.
The applicant has developed a novel shaded-pole single-phase motor, that for a same external area size of the stator of the previous art and smaller stator lamination package width, i.e., by using less material for its manufacture and consequently lighter, supplies the same output power with a smaller electric energy consumption.
On the other hand, there are different techniques for the manufacture and design of motors that provide a motor with more efficient results, but which are more expensive to manufacture such as permanent capacitor motors. Because of its design, the motor of the present invention presents a smaller electric resistance on its coils as well as the use of waste material when punching the magnetic circuit; therefore, it allows providing a less expensive and more efficient motor than the shaded-pole motors of the previous art.